Transcranial magnetic stimulation (TMS) is a non-invasive technique that is used clinically and as an investigatory method in many research and therapeutic applications. Use of TMS on both normal and pathological brain functions has been studied, and TMS has been used in the treatment of neurological and psychiatric disorders including major depression, schizophrenia, dystonia, autism, pain relief, and chronic migraine. Magnetic fields can penetrate and induce electric fields in tissue of other types, as well, tissues such as muscle, liver, and kidney.
Traditionally, TMS uses brief, intense pulses of electric current delivered to a coil placed adjacent to the subject's head. During the pulses, changes in the magnetic field internal to the target brain tissue create resultant electric fields within the brain via electro-magnetic induction. The induced electric field modulates the neural transmembrane potentials and, hence, neural activity. The focus of activation in the brain is approximated by the volume where the induced electric field is nearly maximal. This location depends on stimulating coil field strength, coil geometry, and coil placement.
A dual toroid Figure-8 magnet configuration is commonly used for generating the magnetic field for brain stimulation, but a variety of innovative coil designs are now being proposed and studied.
To map direct neural activity stimulated using TMS, the induced electric field distributions generated by different coil designs have been characterized by theoretical calculations, numerical simulation models, and measurements of the electric currents induced in phantoms or in vivo. Analytical studies have used idealized circular and Figure-8 coil geometries. Only a few commercial coils have been modeled in computational analyses. Thus, field distribution data for many commercial, experimental, or proposed TMS coil designs remain unavailable. Knowledge of electric field spatial distributions generated with specific coil designs and how these fields compare with those generated by alternative coil designs is valuable in the design and interpretation of basic research and clinical studies. Indubitably, the development of novel coil design has been inhibited by the lack of theoretical comparison of the efficacy of presently available or proposed designs.
The two most salient electric field spatial considerations with respect to TMS are depth of penetration (especially field attenuation with depth and with respect to orientation of the coil's major axis) and focality—the ability of a chosen coil to focus (concentrate) a magnetic field deep within the subject tissue. Actual proposed or implemented coil designs have been developed with the objective of improving one or both of these field characteristics. All designs require a tradeoff between attenuation with depth and focality. Focality is important in attempting to target small volumes while simultaneously avoiding similar effects in adjacent non-targeted volumes.
There has also been substantial interest in direct, non-invasive stimulation of brain volumes deeper than the superficial cortex, but electric fields in such deep brain targeting capability is limited by the rapid attenuation with penetration depth. Fields from larger coils penetrate deeper but have reduced focality. Reduced focality is a serious limitation to both clinical and basic neuroscience applications because stimulation of non-target brain regions may affect clinical outcomes, and certainly affect the degree to which any observed changes in behavior can be attributed to stimulation alone. A constant background fear by researchers or clinicians has been associated with understanding and controlling electric field depth and focality. This fear concerns an increased risk of accidental seizure and other adverse side effects.
The variety of types of magnetic fields which might be of interest in the study of magnetic brain stimulation has been described by Deng et al. In these studies a spherical saline-filled phantom was used to model the human head.
Additional relevant information may be found in “Three-dimensional distribution of the electric field induced in the brain by transcranial magnetic stimulation using Figure-8 and deep H-Coils,” by Roth, Amir, Levkovitz, and Zangen in the Journal of Clinical Neurophysiology (2007 February; 24(1):31-8); “Coil Design Considerations for Deep-Brain Transcranial Magnetic Stimulation (dTMS),” by Deng, Peterchev, and Lisanby, 30th Annual International IEEE EMBS Conference, Vancouver, British Columbia, Canada, Aug. 20-24, 2008; and “Electric field depth-focality tradeoff in transcranial magnetic stimulation: Simulation comparison of 50 coil designs,” in Brain Stimulation 6 (2013) 1-13 Brainsway, 19 Hartum Street, Bynet Building, 3rd floor, Har Hotzvim, Jerusalem 9777518, Israel.
The Brainsway reference provides a wide-ranging description of a whole family of coils called Hesed (H) coils that have been proposed to achieve effective stimulation of deep brain structures. The Hesed coils have complex winding patterns and larger dimensions compared to conventional TMS coils and consequently can be expected to have reduced electro-magnetic field depth attenuation and reduced focality. It has been proposed to use high-permeability ferromagnetic cores to improve the electric efficiency, field penetration, and focality of Hesed coils.
In short, there has long been interest in transcranial magnetic stimulation but as yet no effective way to produce controlled, predictable and safe stimulation deep inside the brain. A device that would achieve these goals would be advantageous in research and in medical treatment.